The invention concerns a fuel cell system as well as a method for operation of a fuel cell system, having an anode chamber and a cathode chamber which are separated from each other by a proton conducting membrane, wherein, during an operational state, a fuel can be introduced to the anode chamber and an oxidant, in particular oxygen, can be introduced to the cathode chamber. The invention also concerns a system for interruption-free power supply to at least one electrical user whose energy is normally extracted from an alternating current power network and, in the event of failure of the alternating current power network, energy can be extracted from a fuel supply system. The invention also concerns a method for operating the system.
German patent application P 195 38 381 describes a system for interruption-free power supply to electrical users with which, in the event of power mains failure, a so-called PEM fuel cell (polymer electrolyte membrane) takes over power supply to the user. Towards this end, inlets introduce fuel and an oxidant to the fuel cell. Valves are disposed in these inlets which are closed in the standby state of the fuel cell when the alternating current power network is functioning. During the standby state of the fuel cell, no fuel and no oxidant gains entrance into the fuel cell. Should the power network fail, the valves are opened and the fuel and oxidant are introduced into the fuel cell. The fuel cell is then transferred into an operational mode. In this operational mode, the fuel and the oxidant react in the fuel cell to produce electrical energy.
The transition from the standby state into the operational state of the fuel cell is therefore effected with the assistance of valves. These types of valves, in particular electromagnetically operated valves, have a response time of at least approximately 100 ms. Power network failure can therefore only be compensated for following an interruption time of approximately 100 ms.
It is the underlying purpose of the invention to create a fuel cell system as well as a method for operation of a fuel cell system and a system for interruption-free power supply with which a downtime of less than 100 ms can be achieved.
This purpose is achieved in accordance with the invention with a fuel cell system or a method of the above mentioned kind in that, in the standby state, the oxidant is present in but does not flow through the cathode chamber. The oxidant thereby preferentially exercises pressure on the membrane.
The oxidant is therefore also present in the cathode chamber in the standby state when the alternating current power network is functioning. When the power network breaks down it is therefore not necessary, as was the case in prior art, to first open a valve in order to introduce the oxidant into the cathode chamber. Rather, the oxidant is already present in the cathode chamber and the fuel cell system can therefore take over current supply to the user without delay.
The invention therefore facilitates downtimes between the breakdown of the alternating current power network system and takeover by the fuel cell system which are substantially less than 100 ms. The fuel cell system in accordance with the invention can therefore preferentially be used in a system for interruption-free power supply to electrical users.
In a preferred embodiment of the invention, the cathode chamber is connected to a cathode outlet having a blocking member, in particular a magnetic valve, which is closed in the standby state. In this manner, the cathode chamber can be closed in the standby state at least one side so that the oxidant is present in but cannot flow through the cathode chamber. In the operational state, the blocking member is opened so that the oxidant can then flow through the cathode chamber. Continuous reactions between the fuel and the oxidant then occur.
In a preferred embodiment of the invention, the cathode chamber is connected to a first cathode inlet which is connected to at least one tank, filled with oxidant or the like, via a blocking member, in particular a magnetic valve and/or a pressure reducer. This represents a particularly simple and economical method for making the oxidant available during the standby state.
In an additional advantageous embodiment of the invention, the cathode chamber is connected to a second cathode inlet which is connected, via a blocking member and preferentially a magnetic valve, to a compressor or the like which intakes a gas, preferentially air. The oxidant, in particular oxygen, must not thereby be extracted from the tank during the operational state, rather can easily e.g. be extracted from the air. The oxidant is therefore initially taken from the tank and introduced into the cathode chamber and Subsequent thereto, for prolonged operation, a gas, in particular air, is suctioned into the cathode chamber. The oxidant contained in the tank is therefore not used-up during the operational state of the fuel cell system so that a filling up or an exchange of the tank is only rarely required.
In a particularly preferred embodiment of the invention, the fuel is present in the anode chamber during the standby state. The fuel preferentially exercises pressure on the membrane. Towards this end, it is possible for the fuel to either be statically disposed in the anode chamber, e.g. in the form of hydrogen from a pressure vessel, or the fuel, e.g. a liquid fuel can flow in intervals or continuously through the anode chamber. It is only important that the fuel be present in the anode chamber at the membrane. Therefore, the fuel is also present in the anode chamber during the standby state when the alternating current power network is functioning. When the power network breaks down, it is not necessary, as was the case in prior art, to initially open a valve to introduce the fuel into the anode chamber. Rather, the fuel is already present in the anode chamber and the fuel cell system can therefore take over current supply to the user without any delay.
In a particularly preferred embodiment, the fuel cell is maintained at an optimal operating temperature in the standby state. The power capability of the fuel cell at 80 to 100xc2x0 C. is approximately twice that at room temperature (20 to 30xc2x0 C.). This can be effected by temperature controlling a circuit having liquid fuel or with a separate temperature controlled circuit. Heating is effected by the power mains. This measure improves the instantaneous efficiency of current delivery in the event of network failure. In this manner, the number of cells (stack) can be substantially reduced, which is definitive for investment costs.
The method in accordance with the invention therefore introduces a fuel cell system which, in the standby state with functioning alternating current power network, has a cathode chamber closed at at least one side, but filled with an oxidant so that the oxidant is present in the cathode chamber. The anode chamber is filled with fuel. As a result, the fuel cell system in accordance with the invention produces an off-load voltage in the standby state.
Since the cathode chamber has no through flow in this state, the fuel cell system can only deliver current for a short period of time when loaded, e.g. after a power network failure. One overcomes this situation by opening the blocking member of the cathode chamber during the transition from the standby state into the operational state. The cathode chamber is thereby no longer closed-off and the oxidant can flow through the cathode chamber. In this manner, continuous electrochemical reactions can occur in the fuel cell so that current can be continuously produced. In this operational state, the fuel cell system can then replace the broken down alternating current power network. An H2/O2 cell of approximately 1500 l delivers a power of 250 kW at 80xc2x0 C. over a period of several hours with low (less than 2 bar) sound levels and substantially without pollutant emission.
The amount of time required to open the blocking member assumes values of approximately 100 ms for electromagnetically operated valves. This response time of approximately 100 ms does not however present a problem to the invention, since sufficient reactions can already occur during this time. In prior art, the system did not allow reactions during the time when the valve was being opened. The system in accordance with the invention delivers current within 10 ms.
By exercising pressures in the cathode chamber and the anode chamber which are preferentially of equal size and e.g. assume values of approximately 2 bar, no pressure difference is present across the membrane so that no damage to the membrane can occur.
In an advantageous improvement of the invention, the anode chamber is connected to an anode circuit for introduction of a liquid fuel (e.g. methanol). It is particularly advantageous when this anode circuit comprises a pump and a heater. The fuel can thereby be caused to flow through the anode chamber in a particularly simple manner. In addition, the fuel cell can be easily maintained in the standby state at a desired temperature.
In an advantageous embodiment of the invention, the anode circuit is pressurized. The fuel thereby exercises a permanent pressure on the membrane. This improves the reactions between the fuel and the oxidant such that the fuel cell system in accordance with the invention can switch from the standby state into the operating state in a particularly rapid fashion. In addition, the pressure exercised by methanol fuel in the anode circuit substantially reduces losses due to carbon dioxide discharge.
In an advantageous improvement of the invention, the anode chamber and cathode chamber are accommodated in a gas-tight and optionally additionally heat-insulated housing. In this manner, one prevents the temperature of the fuel in the anode circuit from being substantially influenced by external factors and is therefore reduced to only an insignificant extent, in particular during the standby state. It is particularly advantageous when the housing is pressurized, in particular subjected to nitrogen pressure. This substantially suppresses leakage from the anode chamber and/or the cathode chamber. In addition, the nitrogen pressure prevents boiling of a liquid fuel in the anode circuit, in particular boiling of a methanol/water mixture.